1. Field of the Invention
The present invention relates to a disk producing method for a photo film cassette. More particularly, the present invention relates to a method of producing a plastic disk with high quality to be used in a photo film cassette, of which a leader of the photo film is exited by rotation of a spool in an unwinding direction.
2. Description Related to the Prior Art
U.S. Pat. No. 4,423,943 discloses a type of photo film cassette, of which a leader of unexposed photo film is pre-contained in the cassette shell, and advanced to an outside of the cassette shell when a spool is rotated in an unwinding direction. The spool is constituted by a spool core and two disks, which are disposed on the spool core, for contact with the ends of a roll of the photo film, to render the turns of the photo film neat. In the cassette of the leader-advancing type, it is necessary to transmit rotation of the spool to the roll of the photo film. To this end, the disks of the spool are provided with respective ring-like lips formed on their periphery and projected toward one another. The ring-like lips are located to cover edges of the outermost turn of the photo film, and prevent the roll from being loosened. Such a cassette is suggested in U.S. Pat. Nos. 4,834,306, 4,848,693 (corresponding to JP-A 2-18545), U.S. Pat. No. 5,271,577 (corresponding to JP-A 3-37645), and U.S. Pat. No. 5,407,146 (corresponding to JP-A 3-37645).
To advance the leader of the photo film, it is necessary to spread both disks in the vicinity of a photo film passageway, to release the ring-like lips from regulation. The disks are rotatable, and because they are thin, are usually deformed when they are rotated.
There is a suggestion of a forming method in U.S. Pat. No. 5,211,348 (corresponding to JP-A 4-251841 and 5-119436). A thin, continuous thermoplastic resin sheet is heated. The continuous sheet is deformed in accordance with vacuum forming. The continuous sheet is moved into a punch/die set, where a punch device and a die device cut a circular contour and a bearing hole at each disk-like portion, to form the disks. This known method has an effect of producing a thin disk with high efficiency.
To raise productivity, multiple disks should be cut out at one time precisely. It is important for disks to have precision in size of both its contour and its bearing hole. If a bearing hole of a disk is off-centered with respect to the contour, the disk, when contained in a cassette, cannot resist rotating in eccentric fashion about a spool core. It is then likely that the photo film leader will not advance to the outside, the disk receives excessive load and is broken, and is contacted on the disk contacts the cassette shell and is resistant to advancement of the leader.
It is also important in the disk producing method to set the position of the sheet in effecting the cutting precisely the same as the position in having effected the vacuum forming. If a bearing hole of a disk is off-centered with respect to an inner face of the ring-like lip, there may occur breakage of the disk, and high resistance to advancement of the leader. To overcome such difficulties, there is a suggestion in JP-A 61-51570: positioning holes are formed in the sheet at first. Regulating pins are inserted in the positioning hole to position the sheet, while the sheet is treated in the pressing forming, which is followed by a punching step.
In the disk production method discussed above, it is difficult to remove the punched piece from the sheet. There is a suggestion of a so-called "push-back method" in JP-A 63-196850, in which a punched piece is moved back into a punch hole formed in the sheet, which is transported to next station, where the punched piece is removed from the sheet.
A problem in the method of U.S. Pat. No. 5,211,348 (corresponding to JP-A 4-251841 and 5-119436) lies in low productivity of disks from a sheet. It is general in consideration of productivity that multiple disks should be treated at the same time both during the vacuum forming and during the punching. However a base plate at the base of a punch device should have a larger size than the punch device to keep precision in the punching. Hence, the interval between two adjacent punch devices can only be reduced by a certain amount. There is also a problem with cost. Resin for the disks which has high rigidity, high resistance to heat, and high resistance to flexural fatigue is somewhat expensive. Due to these factors, it is difficult to make a disk less expensive without increasing the productivity of the disc manufacture.
The method of JP-A 61-51570 also has shortcomings. The positioning holes operate to position the sheet in unchangeable fashion irrespective of the pressing forming. There may occur distortion in size of the sheet due to the pressing. The punching may be effected without consideration of the distortion. Punched pieces are likely to have various dimensions.
In the "push-back method " of JP-A 63-196850, it is difficult in stably retaining a thin piece inside the punch hole in the sheet. Punched pieces may be dropped from the sheet during the transport toward the removing station. Another problem is derived from the ring-like shape of the disk with a bearing hole. In particular is difficult or laborious to separate a scrap portion at the bearing hole from a ring-like piece of disk.